The present invention relates to a delay fluctuation absorbing device and method and, more particularly, to a delay fluctuation absorbing device and method used on the reception apparatus side when speech communication is performed by using an Ethernet, ATM (Asynchronous Transfer Mode), or the like.
Conventional apparatuses for transmitting/receiving speech information upon packeting it have used a technique of maintaining continuity in speech communication by starting to read a received packet with a long transmission delay after a lapse of an allowable period of time by using a buffer for absorbing transmission delay fluctuations on the receiving side.
The following problems are, however, posed in the conventional technique. According to Japanese Patent Laid-Open No. 64-29141 (reference 1), the read timing of a transmission delay fluctuation absorbing buffer is fixed depending on the device using the buffer. If, therefore, the read timing is set to be early, the information of a speech packet with a long transmission delay cannot be reconstructed. If the read timing is set to be slow, speech information can be continuously reconstructed, but echoes are produced or talkers feel incongruous in two-way speech communication because the fixed delay time is long.
In order to solve such problems, Japanese Patent Laid-Open No. 9-200265 (reference 2) has been proposed. According to this conventional technique, a packet is transmitted after transmission time information is added to the header portion of the packet on the transmitting side. On the receiving side, the transmission delay time obtained from the transmission time information of the header portion of the packet and reception time information in a receiving unit is compared with the read timing of the transmission delay fluctuation absorbing buffer, and the read timing is changed on the basis of the comparison result. According to this scheme, the read timing of the transmission delay fluctuation absorbing buffer can always be adjusted in accordance with the transmission delay fluctuations.
If, however, speech packets with long and short transmission delays are alternately received, the read timing of the transmission delay fluctuation absorbing buffer is frequently updated, and the difference between the timing before updating and the timing after updating is large. For this reason, speech communication is interrupted at the instant when the timing is changed. In addition, the reception delay time alternately prolongs and shortens. This makes talkers greatly feel incongruous.
A technique similar to that in reference 2 is also disclosed in Japanese Patent Laid-Open No. 2-63346 (reference 3). Other examples of this type of delay fluctuation absorbing device are disclosed in Japanese Patent Laid-Open Nos. 1-248726 (reference 4), 2-67847 (reference 5), 2-203641 (reference 6), and 6-46080 (reference 7).
According to reference 3, a transmission delay time is obtained from a generation time added to a header at the time of transmission and a packet reception time. A received packet is then delayed and reconstructed. According to reference 4, a transmission packet is delayed by a time set therein and read on the receiving side. In reference 5, when the number of packets stored in a FIFO (First-In First-Out) buffer becomes a predetermined value or less, reading of packets from the FIFO buffer is stopped. When this number becomes a predetermined value or more, reading of packets from the FIFO buffer is resumed.
According to reference 6, a change in average delay time is monitored. When the change ratio is a predetermined value or less, and fluctuations are not absorbed, the fluctuation absorption delay time is prolonged. When fluctuations are absorbed, the fluctuation absorption delay time is shortened. In reference 7, the basic fluctuation absorption time is corrected on the basis of the number of cells received within a predetermined monitoring time T upon arrival of the first cell, and a decoding time.
In any of references 3 to 7, however, any technique of solving the above problems is not disclosed.
It is an object of the present invention to provide a delay fluctuation absorbing device and method which can improve speech quality.
It is another object of the present invention to provide a delay fluctuation absorbing device and method which can make a talker feel less incongruous in hearing.
In order to achieve the above objects, according to the present invention, there is provided a delay fluctuation absorbing device comprising buffer means for storing a packet sent through a transmission line, and buffer control means for reading out the packet from the buffer means a predetermined read delay time after the packet is stored in the buffer means, and updating the read delay time of the buffer means on the basis of a transmission delay time of the packet at unit time intervals, wherein the buffer control means shortens the read delay time step by step when the read delay time is to be shortened by updating.